I hiked S. Bass to Tanner along the Tonto in 2000. It's about 105 miles. I think we started late February. Past April,water can get tricky. March is considered prime canyon season. It was 5 F on the S rim at night, but once you get into the canyon at the Tonto plateau level it maybe only got down to freezing at night. We had one week's food muled down to Phantom Ranch and reservations at the dorms to get cleaned up, eat some real food, and re-pack. We weren't going UL, maybe 35-40 lb packs. We had 2.5 lb down bags, a 4 lb tent, hard shell goretex pants and jackets, polarguard or down jackets. We slept out of the tent all but maybe 2 nights.
The primitive road into S. Bass can be tricky (mud, frozen) so we prefer to start there getting dropped off by someone with 4WD. The Escalante route east of Hance where the Tonto trail peters out is also easier to follow after you had time in the canyon. If you start at Tanner, getting onto the Escalante route is confounded by river rafters trails.
I know people who have through hiked the Tonto in 9 or 10 days. I suppose higher mileage is possible, but going down hill the first day usualy takes its toll, and the trails can be primitive and hard to follow in spots. Also, below the rim all camping is permitted, and the park narcs will fine you if you camp off permit either ahead or behind schedule. The back country rangers will also discourage issuing permits that have long days. They get a lot of unprepared people lured by the easy downhill walk, and tend to assume backpackers don't know what they are getting into.
I recommend trelling poles for the canyon. Their use on the first day going in is worth it alone.